For information on placing a window of type DIALOG wherever you like, see the article by Ken Lewis, How to Place a Dialog.

If you simply want to center a dialog, you can do it with the STYLEBITS command, placing the _DS_CENTER stylebit as the first argument, so that it is added to the style of the dialog. Note that this only works with DIALOG windows!

nomainwin
stylebits #1, _DS_CENTER,0,0,0
open "Centered Dialog" for dialog as #1
#1 "trapclose [quit]"
wait
[quit] close #1:end